Topic: CB 750 can’t rev beyond 6500 rpm

[jmartingrenier]

1981 Honda cb750.
Starts fine, pulls great and strong right up to 6500 rpm then just sits there and stumbles even though there is plenty of throttle left.
Nee wires, plugs, fuel lines, all carbs removed cleaned and replaced. Stock air box, stock muffler.
Coils both pull 3ohms. New voltage regulator.

This one has me stumped!

[69cb750]

I would check point gap and timing light to verify full advance.
Fuel, compression, ignition, my guess is ignition.

[scottly]

I would check point gap

No points on a 1981 CB750.

[scottly]

Does it pull to 6500 whatever gear you are in, or only in first or second?

[HondaMan]

When you had those carbs apart...did you check to make sure they had genuine Keihin jets, needles, and needle jets?
And...what depth were those floats set for?
Do your carbs have the air-inlet diaphragms on their sides (usually the left side)? And, were any of the diaphragms cracked?

How is the fuel-feed hose to the carbs routed?

[scottly]

What is the history on the bike? Is it new to you, and always ran this way since you've had it, or did it once run fine and this is something new?
You might want to check the temperature of all four exhaust pipes near the head right after starting the bike: the pipes should get hot enough to make spit sizzle. If one or two pipes only get warm, not sizzling hot, those cylinders are not firing.